Remove multithreading for sorting

The relevant processes are so fast, mutexes and mutex locks are so
expensive, and iterators so efficient, that it's actually faster to run
single-threaded across all the data than to spin up a bunch of threads
and have them basically spinlock waiting for the global mutex involved
either directly or in a channel.

ivy_files(kubernetes) time: [10.209 ms 10.245 ms 10.286 ms]

change: [-36.781% -36.178% -35.601%] (p = 0.00 < 0.05)
Performance has improved.

ivy_match(file.lua) time: [1.1626 µs 1.1668 µs 1.1709 µs]

change: [+0.2131% +1.5409% +2.9109%] (p = 0.02 < 0.05)
Change within noise threshold.